This invention generally relates to valve plates for use in slide gate valves for controlling a flow of molten metal, and is specifically concerned with a valve plate assembly that is resistant to cracks caused from thermal stresses.
Slide gate valves are commonly used to control a flow of molten metal in steel making and other metallurgical processes. Such valves comprise a support frame, an upper stationary valve plate having an orifice in registry with a tundish or ladle nozzle for conducting a flow of molten metal, and a throttle plate likewise having a metal conducting orifice that is slidably movable under the stationary valve plate. In slide gate valves used in conjunction with continuous casting molds, a lower stationary valve plate is provided beneath the movable throttle plate which likewise has a flow conducting orifice that is substantially aligned with the orifice of the upper stationary plate. The rate of flow of molten metal is dependent upon the degree of overlap of the orifice of the slidably movable throttle plate with the orifice of the upper stationary plate. The movable throttle plate is usually longer than the stationary throttle plates in order to give it the capacity of throttling the flow of molten metal from both the front and back edges of its own orifice, as well as the ability to shut off the flow altogether by bringing its orifice completely outside of any overlap with the orifices of the stationary plate. Typically, the throttle plate is slidably manipulated between the stationary plates by means of a hydraulic linkage.
Both the throttle plate and the stationary plates of such slide gate valves are formed from heat and erosion resistance refractory materials, such as aluminum oxide, alumina-carbon, zirconium oxide. However, despite the heat and erosion resistance of such refractory materials, the severe thermal stresses that they are subjected to ultimately causes some degree of cracking to occur. For example, in steelmaking, each valve plate is subjected to temperatures of approximately 2900.degree. in the area immediately surrounding its flow-conducting orifice, while its exterior edges are experiencing only ambient temperature. The resulting large thermal gradient creates large amounts of mechanical stress as the area of each plate immediately surrounding its orifice expands at a substantially greater rate than the balance of the plate. These stresses cause cracks to form which radiate outwardly from the orifice of the plate. If nothing is done to contain the spread of these cracks, they can extend all the way to the outer edges of the plate, causing it to break.
To prevent the spreading of such cracks and the consequent breakage of the valve plates, various clamping mechanisms have been developed in the prior art. The purpose of these mechanisms is to apply sufficient pressure around the perimeter of the plate so that cracks emanating from the orifice do not spread to the edges of the plate. In one such mechanism, a steel band is stretched around the perimeter of each of the valve plates. Unfortunately, the applicants have observed that there are at least three disadvantages associated with the use of such band-type clamping mechanisms. First, because the steel that forms such bands is a superior thermal conductor to the air that would otherwise surround the plate edges, the use of a steel band actually increases the thermal gradient across the lengthwise and widthwise axes of plate, thereby encouraging even more cracking to occur. Secondly, as the steel band heats up as a result of being in the vicinity of molten metal, it expands much faster than the refractory material forming the valve plates, which in turn causes it to relax the compressive forces that it needs to apply around the plate in order to discourage the spread of cracks. Thirdly, if the corners of the plate are not rounded, such clamping bands can apply localized mechanical stresses onto the corners of the plates, which in turn can cause unwanted cracking in these areas.
To overcome these and other shortcomings, clamping systems have been developed that comprise a frame having screw-operated wedges which engage corners of the plate that have been truncated in an angle that is complementary to the angle of the wedges. While such frame and wedge type clamping mechanisms constitute a clear advance over the mere use of steel banding around the perimeter of the plates, the inventors have further noted at least two shortcomings with this design that prevent it from achieving its full, crack-retarding potential. In all of the variations of this design that the applicants are aware of, the angle of each of the truncated corners with respect to either the lengthwise or widthwise edge of the plate is the same, regardless of the position of the orifice along the longitudinal center line of the plate. Consequently, in plates where the orifice is offset along the longitudinal center line of the plate (which includes virtually all valve plates), the clamping forces cannot be uniformly focused where the maximum amount of cracking occurs, i.e., in the vicinity of the orifice where the greatest amount of thermal stresses are present. Moreover, even in instances where the orifice is centrally located in the valve plate, the applicants have observed that the angular orientation of the truncated corners in such plates does not optimally prevent the spreading of cracks, as previously thought. Such non-optimality results from the face that crack formation is not uniformly distributed 360.degree. around the orifice, but instead is biased along the longitudinal center line of all valve plates whether stationary or movable. Such an asymmetrical distribution of cracks around the plate orifices is believed to occur as a result of the longitudinal sliding action of the throttle plate across the faces of the stationary plates. Still another shortcoming associated with prior art clamping mechanisms is their use, in some cases, of angles shallower than 20.degree. with respect to the longitudinal edges of the plate. In addition to providing inadequate clamping forces to close up cracks along the transverse axis of the plate, the use of such shallow angles generates large localized stresses due to the large amount of compression that the clamping wedges apply to the truncated corners. Such localized stresses can result in cracking and fissuring in the corner regions of the valve plates, which is directly contrary to the overall purpose of the clamping mechanism. A final shortcoming associated with such valve plates in general is their lack of any optimization of the length of the truncated corners, or the lengths and widths of the plate with respect to the diameter of its orifice. While the corner lengths should be of a certain minimal size in order to avoid the production of unwanted localized mechanical stresses in these regions of the plate, they should not be made overly large, either.
Clearly, there is a need for a valve plate whose corners are truncated at angles that optimally focus the clamping forces in the most crack-prone areas of the plate in order to maximally retard the lengthening of any such cracks. Ideally, the corners should have a length sufficient to avoid the production of unwanted localized mechanical stresses in the corners.